Well, the net result of the knee mill experiment in the absentee thread was that moving the knee requires a big servo motor and a big drive amplifier to feed it. Another option is to somehow counterbalance the weight of the knee.
Having used retrofitted cnc mills with Z on the quill, I can say they are not too bad. The biggest pain is not having a high enough work envelope for some jobs. Mixing long drill bits with short stubby endmills in a job requiring 4" of Z depth just doesn't work
Moving the knee is not necessarily more accurate than moving the quill. This is because on a manual knee mill, the machinist locks the knee for rigidity. Locking is not going to be feasible for a knee Z drive.
Making a good connection to your quill for a quill Z drive can sometimes be a challenge too. Whether you have to machine some surfaces to make a mounting plate on the quill or the knee, it is almost mandatory that you have a shop nearby with a working mill that you can use to modify your own.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)